Back in the 1830s, a Scottish minister and amateur astronomer named Thomas Dick tried to calculate the number of intelligent creatures in the universe. He assumed that all heavenly bodies supported intelligent life, maybe not exactly like us, but similar to us in size and habits of living. Then he took population figures for Great Britain and, assuming that space aliens lived just as densely, he projected populations onto various planets.

There are, he decided, 50 billion Venusians living on Venus.

Mars, he thought, had 15 billion Martians.

22 Trillion Times 31

Jupiter? Seven trillion Jupiterinos — or whatever you call them. He even thought that Saturn's rings were totally occupied by 8 trillion inhabitants — on the rings alone! In the end, he figured our solar system was home to 22 trillion individuals, and that, he said, did not include the sun. The sun, he thought could support an additional 31 times as many creatures — because it only seemed sensible that every celestial orb was, in effect, a floating shelter for somebody. Everything you could see in the sky was a home.

A generation earlier, the brilliant astronomer William Herschel (discoverer of the planet Uranus) felt pretty much the same way. In 1794, he also said the sun was probably inhabited, "like the rest of the planets, by beings whose organs are adapted to the peculiar circumstances of that vast globe." Somehow, sun-dwellers had learned not to boil.

Itty Bitty Life

That was then. Today we are in a very different mood. We have ceased to expect any life form that's intelligent (or even large) in our solar system, other than life here on Earth. We spend our exploratory dollars painstakingly searching for little bits of microbial life or, failing that, we hope to turn up a rare fossil remnant of a life that blinked out hundreds of millions of years ago. Instead of Thomas Dick's universe jampacked with creatures, we are even imagining the radical alternative, that there is nobody anywhere — except for us.

If that's too depressing (or improbable), then there's the thought that if intelligent life exists elsewhere, it is so remote, so hard to find, we may never make contact. In the end, we may never know for sure if we are unique, extraordinary or commonplace. We just won't know. Ever.

That's a sad falling off from the exuberance of the 1830s. But stick around. The mood, says Columbia University astrobiology professor Caleb Scharf could change — and soon. In his new book, The Copernicus Complex, he addresses the riddle of life in the universe and says, "We are much, much closer to an answer than we have ever been in the history of the human species; we are on the cusp of knowing."

Whoa!

What's changed? Professor Scharf says we now have the tools we need — though I suspect they will have to be refined — to spot life's true colors. And when we look across the universe, that's what we should be looking for, he argues: colors. Telltale colors.

Given the "right instruments," he writes, we will soon be able to target a planet, and look at the light reflecting from its atmosphere (if it has one) and, by reading a spectrograph that tracks colors, we will see, in effect, signs of life.

To oversimplify, let's pretend we see a planet that looks like this ...

And let's say the presence of yellow (I'm making this up) is evidence of oxygen, while the presence of pink is evidence of methane. So this planet has both oxygen and methane floating in its air. So?

So, says Scharf, oxygen and methane are not usually found floating in an atmosphere. They normally combine with other elements and disappear from the air. "Detecting both of these gasses in an atmosphere ... tells us that something must be continually replenishing them, and one of the best sources is life itself." So this planet now has a "biosignature" — essentially a chemical exclamation point that says, "Check me out! I may be a Carrier of Life."

With 1,700 planets already discovered (700 just in the past year), biosignatures give us something specific to look for.

We already use color spectrum technology to map changes on Earth. Satellites use reflected light to track growing and shrinking lakes, deserts, forests, meadows, parking lots, beaches. Low-level plants reflect 10 times the usual near-infrared light. What's happening low on the ground sends different reflections back up to space, and there's hope, writes Scharf, "that as we get better and better at capturing the light from distant worlds ... we may spot these biosignatures."

If a tinge of blue, a hint of green, a splash of infrared catches our eye, that doesn't mean we're glimpsing anything intelligent. After all, for most of Earth's history, the only beings around were (to quoteNew York Times reporter George Johnson) "unicellular slime." Slime may leave a signal, but it isn't fun. Saturnal Ring Beings by the trillions? That's a party.

But once we see slime's telltale colors, then we can narrow our search, look closer, and maybe, just maybe, improve our chances of finding E.T.

If E.T. is out there, he, she, it is probably inhaling, exhaling, creating waste, making noise, building, buzzing about, leaving, as Caleb Scharf says, "a filthy fingerprint" of color which points straight back to its home like a rainbow landing in what would truly be a pot of gold — the secret address of our nearest neighbor.

Caleb Scharf's new book nicely tackles the probability of life in the universe question, arguing that what's improbable/probable depends on what you think life should look like. If you start at the end of the tale with yourself as the star of the show, the chances of you happening do seem very improbable. But what if you loosen up, and imagine countless different forms of intelligent life? Then the odds change. His book is called "The Copernicus Complex: Our Cosmic Significance In A Universe Of Planets And Probabilities."